Not applicable.
The present invention relates to a method and apparatus for converting synthesis gas, i.e., a mixture of carbon monoxide and hydrogen, to alcohols, particularly methanol. Particularly this invention relates to the use of a catalytic distillation reactor to achieve both reaction of the syngas and high net conversion. High net conversion occurs through use of multiple distillation stages within the reactor to achieve net conversion beyond the thermodynamic limit for a single stage.
Large quantities of methane, the main component of natural gas, are available in many areas of the world. Methane can be used as a starting material for the production of alcohols. The conversion of methane to alcohols is typically carried out in two steps. In the first step methane is reformed with water or partially oxidized with oxygen to produce carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted to alcohols.
This second step, the preparation of alcohols from synthesis gas is well know in the art and is an example of carbon monoxide hydrogenation reactions. A variety of reactions can produce alcohols from synthesis gas. Methanol synthesis is a very common reaction. Further, the Fischer Tropsch reaction also produces alcohol by-products. The Fischer-Tropsch reaction conventionally involves the catalytic hydrogenation of carbon monoxide to produce a variety of products ranging from methane to higher alkanes. Catalysts for use in synthesis of these various products from synthesis usually depend on the desired product. Catalysts for the production of hydrocarbons usually contain a catalytically active metal from one of the Groups 8, 9, or 10 (in the New notation of the periodic table of the elements, which is followed throughout). Group 8, 9, and 10 metals have also been used in catalysts for the production of alcohols, catalysts for the production of alcohols. However, catalysts for the production of alcohols, particularly methanol, typically are copper-based, many containing copper in the form of an alloy, such as copper-zinc alloys and copper-rare earth alloys. The catalysts may additionally contain one or more promoters. Promoters for copper-zinc catalysts include Cr, Al, Mn, V, and Ag, among others.
Traditional methods of Fischer-Tropsch synthesis produce a range of products. In a methanol synthesis process, by-products may include hydrocarbons, higher alcohols, dimethyl ether, esters, ketones, and aldehydes. The range of hydrocarbons based on the carbon chain length of the hydrocarbon is discussed in U.S. Pat. No. 4,619,910, which is incorporated herein by reference. This well-known distribution is known as the Anderson-Schulz-Flory distribution. In general, the range of hydrocarbons produced in Fischer-Tropsch processes may be characterized by the Anderson-Schulz-Flory distribution with a suitable value for the parameter alpha, regardless of catalyst type.
Because of the range of products, typical systems that use the Fischer-Tropsch process provide a separation stage that follows the reaction stage. The separation stage is often one or more distillation columns. The distillation columns separate the product into fractions according to boiling point. The lighter products, having lower boiling points, will vaporize and pass to the overhead region of a distillation column, where they can be removed as one product stream. The heavier products, having higher boiling points, will condense and fall to the lower region of the distillation column, where they can be removed as a separate product stream. In addition, any one or more of the product streams having intermediate compositions can be removed from the column at intermediate points between the top and the bottom and may then be sent to other columns for further separation if desired. In this way, in a process for producing methanol, the methanol may be separated from undesired by-products.
Water can also be also produced during Fischer-Tropsch synthesis. Recent research indicates that water can deactivate a Fischer-Tropsch catalyst in certain circumstances. Rothaemel, Hanssen, Blekkan, Schanke and Holmen, The Effect of Water on Cobalt Fischer-Tropsch Catalysts Studied by Steady-State Isotropic Transient, Kinetic Analysis, 38 Catalysts Today 79-84(1997); Schanke, Hilmen, Bergene, Kinnari, Rytter, Adnanes and Holmen, Reoxidation and Deactivation of Supported Cobalt Fischer-Tropsch Catalysts, Energy and Fuels, Vol. 10 No.4(July/August 1996) p. 867-872.
In addition, the catalytic methanol synthesis as with the Fischer-Tropsch synthesis, when practiced on a commercial scale, generates heat that must be removed from the reaction vessel. Methanol and Fischer-Tropsch synthesis reactions are highly exothermic, and reaction vessels must be designed with adequate heat exchange capacity. Large scale reactors, which potentially offer the economic advantages that come with higher volumes, must presently include, at significant cost, sufficient heat transfer equipment within the reactor to remove the heat generated during the reaction. The traditional method for doing this, and a method that may be used in the present invention, is to place heat removal equipment inside the reaction vessel. A typical internal heat removal arrangement comprises a system of tubes within one or more reaction chambers. The tubes contain a fluid such as water, or any other acceptable fluid, which acts as the heat exchange medium. In operation, the heat generated within the reaction chamber passes through the heat exchange tubes and heats the fluid therein. The heat exchange fluid is then pumped outside the reaction vessel, where the heat is released, preferably through a heat exchanger. This process can be carried out continuously, with the heat exchange fluid circulating through the reaction chamber. A shortcoming of the internal heat exchange process is that the internal heat exchange tubes occupy reactor space. Internal heat removal equipment may therefore decrease the reactor volume that is available for Fischer-Tropsch synthesis, thus limiting the capacities and efficiencies for a given reactor.
The conversion of natural gas to methanol via syngas is a widely used industrial process. Heat integration and recovery are desirable features of the process. For example, methanol is manufactured in large amounts due to its use in a variety of applications, including: a feedstock for other chemicals, fuel use, and other direct uses as a solvent, antifreeze, inhibitor, or substrate. Further, there is a wide range of more specific uses, as described below.
According to one application, methanol is used as a solvent in automobile windshield washer fluid and as a cosolvent in various formulations for paint and varnish removers. It is also used as a process solvent in chemical processes for extraction, washing, crystallization, and precipitation. For example, methanol is used as an xe2x80x9cantisolventxe2x80x9d for precipitation of polyphenylene oxide after its polymerization. It should be pointed out here that there have been active studies in using the extracts of agricultural plants in medicine. Methanol is often used for the extraction. Methanol extracts of some plants show antibacterial activities. This provides a potential use of methanol in traditional medicine.
According to another application, methanol is used as antifreeze because it has a high freezing point depression ability. It depresses the freezing point of water by 54.5xc2x0 C. for a 50-50 wt % methanol-water mixture. The largest antifreeze use of methanol is in the cooling system for internal combustion engines. However, the antifreeze market for methanol has been saturated. Its market share has been lost to ethylene glycol since 1960 because of the superior performance of the glycol.
According to yet another application, methanol finds some use as an inhibitor. It inhibits formaldehyde polymerization and is present in paraformaldehyde. Methanol can also serve as a hydrate inhibitor for natural gas processing.
According to still another application, methanol is an inexpensive source of carbon. For this reason, it is a substrate used in may applications for supplying the energy needed for the growth of microorganisms. For example, single-cell protein, a protein in a variety of microbial cells, is produced through fermentation using hydrocarbon substrates, such as methanol. Methanol is also often chosen as the energy source for sewage treatment.
Further, in addition to present uses, new uses such as alternative automobile fuel, supplemental gas turbine fuel at peak demand of electricity, H2 for fuel cells, fuel and cooling system for hypersonic jets, CO and H2 for chemical processes and material processing involve dissociated methanol.
The trend in methanol production has been toward larger capacity and improved energy efficiency. However, thermodynamic equilibrium conditions limit the obtainable conversions of CO and CO2 to methanol according to Equation 2 and Equation 3.
CO+2H2CH3OH xe2x80x83xe2x80x83(2) 
CO2+3H2CH3OH+H2O xe2x80x83xe2x80x83(3) 
On technically performing methanol synthesis accurate matching of temperature, pressure, concentration and catalyst activity are desirable to obtain maximum yields and optimum economics in view of the conversion limitation imposed by the equilibrium. In order to achieve production beyond the thermodynamic limit, conventional methanol production uses multiple reactors, typically with un-reacted feed from one reactor passed to a following reactor. Methanol is typically separated in still another separator apparatus separate from the reactors.
Demand for methanol as a chemical raw material is rising. Further, methanol may play a significant role as a source of energy in the future. Still further, raw materials are becoming less available and more costly. Thus, it is desirable to provide an improved methanol production process that would obtain a higher yield of desired products in a single reactor.
Thus, notwithstanding the foregoing patents and teachings, there remains a need for a continuous methanol synthesis by which the production of certain alcohols can be maximized and controlled.
The present invention overcomes the deficiencies of the prior art.
The present invention provides a method for producing alcohols from synthesis gas. Particularly, the invention provides the use of a catalytic distillation reactor for methanol or higher alcohol synthesis. In a preferred method, a catalytic distillation reactor is used as a single apparatus to simultaneously achieve both reaction to form at least one alcohol from synthesis gas starting materials and the separation of the alcohol product into various product streams. In a more preferred method, a catalytic distillation reactor is used as a single apparatus to simultaneously achieve both the reaction of synthesis gas and the production of methanol, with a net conversion beyond the thermodynamic limit. Alternately, or in combination, the present method may be used in more general operations to produce other alcohols with similar thermodynamic limitations to methanol.
In a preferred embodiment of the present invention, a process for producing methanol includes contacting synthesis gas with a catalyst in a reaction chamber in a catalytic distillation reactor operating at methanol conversion-promoting conditions. This catalyst can be in a solid or liquid phase. The method may include removing methanol from the reaction chamber. The method may include contacting synthesis gas with catalyst in addition reaction chambers in the catalytic distillation unit. Preferably the contacting occurs in at least three reaction chambers, more preferably at least four reaction chambers, most preferably at least five reaction chambers. Further, the method includes selecting the conversion-promoting conditions in each reaction chamber such that methanol is produced with an optimized yield.
According to some embodiments of the present invention, a method of producing methanol has the advantage of providing net conversion of synthesis gas to methanol beyond the thermodynamic limit, while simultaneously separating methanol from reaction by-products, in a single reaction vessel.
Thus, the present invention comprises a combination of features and advantages that enable it to overcome various problems of methanol production. The various characteristics described above, as well as other features, objects, and advantages, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings.
Other objects and advantages of the invention will appear from the following description. For a better understanding of this invention, reference is made to the detailed description thereof which follows, taken together with the subjoined claims.